Monomer conversion, dimensional stability, strength, modulus, surface apatite precipitation and wear of novel, reactive calcium phosphate and polylysine-containing dental composites

Abstract

PurposeThe aim was to assess monomer conversion, dimensional stability, flexural strength / modulus, surface apatite precipitation and wear of mono / tri calcium phosphate (CaP) and polylysine (PLS)—containing dental composites. These were formulated using a new, high molecular weight, fluid monomer phase that requires no polymerisation activator.Materials and methodsUrethane and Polypropylene Glycol Dimethacrylates were combined with low levels of an adhesion promoting monomer and a light activated initiator. This liquid was mixed with a hybrid glass containing either 10 wt% CaP and 1 wt% PLS (F1) or 20 wt% CaP and 2 wt% PLS (F2). Powder to liquid mass ratio was 5:1. Commercial controls included Gradia Direct Posterior (GD) and Filtek Z250 (FZ). Monomer conversion and polymerisation shrinkage were calculated using Fourier Transform Infrared (FTIR). Subsequent volume increases in water over 7 weeks were determined using gravimetric studies. Biaxial flexural strength (BFS) / modulus (BFM) reduction and surface apatite precipitation upon 1 and 4 weeks immersion in water versus simulated body fluid (SBF) were assessed using a mechanical testing frame and scanning electron microscope (SEM). Mass / volume loss and surface roughness (Ra) following 7 weeks water immersion and subsequent accelerated tooth-brush abrasion were examined using gravimetric studies and profilometer.ResultsF1 and F2 exhibited much higher monomer conversion (72%) than FZ (54%) and low calculated polymerization shrinkage (2.2 vol%). Final hygroscopic expansions decreased in the order; F2 (3.5 vol%) > F1 (1.8 vol%) ~ Z250 (1.6 vol%) > Gradia (1.0 vol%). BFS and BFM were unaffected by storage medium type. Average BFS / BFM upon 4 weeks immersion reduced from 144 MPa / 8 GPa to 107 MPa / 5 GPa for F1 and 105 MPa / 6 GPa to 82 MPa / 4 GPa for F2. Much of this change was observed in the first week of immersion when water sorption rate was high. Surface apatite layers were incomplete at 1 week, but around 2 and 15 micron thick for F1 and F2 respectively following 4 weeks in SBF. Mass and volume loss following wear were equal. Average results for F1 (0.5%), F2 (0.7%), and FZ (0.5%) were comparable but lower than that of GD (1%). Ra, however, decreased in the order; F1 (15 μm) > F2 (11 μm) > GD (9 μm) > FZ (5 μm).ConclusionsHigh monomer conversion in combination with large monomer size and lack of amine activator should improve cytocompatibility of the new composites. High monomer molecular weight and powder content enables low polymerisation shrinkage despite high conversion. Increasing active filler provides enhanced swelling to balance shrinkage, which, in combination with greater surface apatite precipitation, may help seal gaps and reduce bacterial microleakage. High monomer conversion also ensures competitive mechanical / wear characteristics despite enhanced water sorption. Furthermore, increased active filler could help reduce surface roughness upon wear.

Monomer conversion, dimensional stability, biaxial flexural strength, and fluoride release of resin-based restorative material containing alkaline fillers

“…Gravimetric studies were used to assess mass and volume changes of all materials (n=3) 14,21) . Disc specimens were prepared 14) . Materials were prepared according to their manufacturer instruction at room temperature (25±1°C).…”

“…Furthermore, the absorbed water also enables release of ions from hydrophilic/reactive fillers. The water sorption induced expansion of polymer-based materials could be governed primarily by the flexibility of polymer matrix and the addition of hydrophilic components 14,16,21) .…”

“…The volumetric expansion upon water sorption, i.e. hygroscopic expansion, could potentially help to compensate polymerization shrinkage and relieve shrinkage stress of resin composites 13,14) . This may subsequently help to reduce the stress generated at tooth-restoration interface that could lead to gap formation and bacterial microleakage.…”

Long-term water sorption and solubility of experimental bioactive composites based on amorphous calcium phosphate and bioactive glass

“…3a). The contribution of bioactive fillers to water sorption of a composite material 28) depends on the relative hydrophilicity of bioactive fillers and neat resins. For example, the resin system of the ACP-series was extremely hydrophilic and the addition of 40 wt% of ACP fillers increased the water sorption for only 3%.…”

Light-Curing Units, Photoinitiators System, and Monomers on Physico-Mechanical Properties of Experimental Composite Resins

“…But, the incorporation of TEGDMA causes a clinically undesirable increase in polymerization shrinkage, such as decreases general mechanical properties, increases water sorption, and hinders color stability. Other diluent monomers with low viscosity and high molecular weight, such as ethoxylated bis-phenol A dimethacrylate (BisEMA; 540 g.mol -1 and 3 Pa.s), have been developed and introduced in commercial composite resins to overcome the limitations of TEGDMA [2,8,10,11,14].…”